1. Field of the Invention
The invention relates in general to electrical inductive apparatus, such as instrument transformers, and more specifically to new and improved high voltage current transformers.
2. Description of the Prior Art
Protective relaying and metering functions require very precise measurement of the current flowing in electrical power circuits. Current transformers for measuring current flow must be constructed to have a very low exciting current, so the ampere turns in the secondary winding closely match the ampere turns of the primary winding. Current transformers should also be constructed to have as small a leakage flux as possible. In conventional power transformers, the leakage flux is much smaller than the working flux and is thus a negligible factor when calculating exciting current, especially since the exact value of the exciting current is not significant. In the current transformer, however, the leakage flux is of the same order of magnitude as the working flux, and the exact value of the exciting current is of major importance.
When the current transformer is to measure the current flow in high voltage, and extra high voltage (EHV) circuits, the current transformer is subject to the hereinbefore mentioned requirements, and its construction is complicated by the fact that the primary winding must be insulated to withstand the very high voltages involved. While the voltage across the terminals of the primary winding of the current transformer is small, the voltage from the primary winding to ground is the same as the voltage of the circuit whose current is being measured.
A prior art arrangement for obtaining the required current transformer performance is to use a bushing-type current transformer as the secondary winding. The bushing-type current transformer has a low loss ring magnetic core wound from a strip of magnetic material, which construction allows distributed secondary windings or coils to be used. The leakage flux is also kept out of the core in a bushing transformer, and thus does not increase the exciting current. The bushing-type current transformer, however, being a ring-shaped structure, requires a primary winding construction which will accept the "window" type construction. A common prior art primary winding which will accept a bushing current transformer for the secondary winding is U-shaped, and is commonly referred to as a "hairpin" primary. A disadvantage of the hairpin primary is the fact that the high voltage bushing must have a diameter which will accept the spaced legs of the hairpin configuration.
U.S. Pat. No. 3,299,383, which is assigned to the same assignee as the present application, discloses a current transformer structure which is especially useful for EHV, wherein the primary winding is a loop, the ends of which are connected to concentric high voltage leads. The advantages of this arrangement, which arrangement is commonly referred to as an "eye bolt" primary, include the fact that efficient cooling of the primary winding and lead arrangement may be achieved, regardless of the thickness of the solid insulation, and the concentric leads enable a much smaller diameter high voltage bushing to be used, which substantially reduces the cost of the bushing. The magnetic core, however, is constructed of flat metallic laminations which are stacked by hand to provide a four-sided magnetic core structure which encircles the insulated loop of the primary winding. The joints at the corners of the magnetic core introduce core losses, necessitating more core material than would be required in a comparable bushing-type current transformer secondary, and the secondary winding is machine wound and placed on a leg of the magnetic core. Leakage flux, however, will link the legs of the core which do not contain the secondardy winding, and will thus undesirably increase the exciting current which adversely affects the ratio or phase angle of the current transformer. This is especially true due to the clearances required at EHV voltages, which cause a relatively high leakage flux. Thus, it is necessary to add one or more equalizer coils to the stacked core legs which do not contain the secondary winding. The equalizer coils have the same number of turns as the secondary winding, and they are connected in parallel therewith. In EHV current transformers, three equalizer coils are normally used, each of which has the same number of turns as the secondary winding.
With the secondary winding and equalizer coils all connected in parallel, the output voltages of the secondary winding and equalizer coils must be the same. Therefore, the induced voltages in the secondary winding and equalizer coils must be nearly equal, and the flux linking the secondary winding and equalizer coils must be nearly equal. Thus, if leakage flux attempts to flow through the magnetic core, currents will be induced into the equalizer coils which will oppose and divert the leakage flux into the air, thus keeping it out of the core. Therefore, the saving savings the cost of the bushing are just about offset by the larger magnetic core, and the cost of stacking the core, and the cost of the equalizer coils.